1. Field of the Invention
This invention relates to surface-emitting laser diodes with tunnel junctions and fabrication methods thereof, which are used for light sources of optical information processing or high-speed optical communication.
2. Description of the Related Art
In recent years, attention has been attracted to surface emitting laser diodes, in particular, to Vertical-Cavity Surface-Emitting Laser diodes (hereinafter, referred to as VCSEL) in the technical fields of the optical communication and optical storage.
VCSEL has excellent characteristics that are not provided by conventional edge-emitting laser diodes. For example, VCSEL have low threshold current and low power consumption. An optical spot is easily obtainable. The device can be tested at the wafer level. The structure can be integrated in a two-dimensional array configuration. Expectations for VCSEL having the afore-mentioned advantages are raised as a low-end light source in the communications field.
The optical communication with optical fibers has been used in mainly mid- and long-wavelength range data transfer (ranging from several kilometers to several tens kilometers) The conventional optical communication employs the single-mode optical fiber with silica as a material and the distributed feedback (DFB) laser having an oscillation peak in the long wavelength range at 1.31 μm or 1.55 μm. The laser in this long-wavelength range exhibits excellent characteristics of “lower dispersion and extremely smaller transmission loss”. Nevertheless, the wavelength has to be controlled strictly. This demands temperature control of the device and causes a problem that downsizing is difficult. In addition, the production volume of the optical fibers is still small, as compared to the consumer products, largely because only telecommunications carriers are the users of the optical fibers. Therefore, the laser diodes are considered costly device elements.
These days, owing to a growing rate of the Internet access with asymmetric digital subscriber line (ADSL) or cable TV (CATV) at home, it is possible to transmit large-volume data as much as dozens of times, much larger than ever before. Along with the popularization of the Internet access, demands on the large-volume data transmission will be further increased. The optical fibers will definitely be available at home in the near future.
In the short-distance communication (ranging from several meters to several hundred meters), however, it is too expensive to use both the single-mode optical fiber and the DFB laser. It is considered economical to employ a combination of a costly optical fiber such as multimode silica fiber or plastic optical fiber (POF) together with a short-wavelength range (at 0.85 μm) that gives good performance when used with the afore-mentioned fibers. VCSEL is becoming widely used for the above-described applications.
In the mid- and long-distance communication, on the other hand, the single-mode optical fiber and the DFB laser are still used. However, the demands for cost reduction in the mid- and long-distance communication will apparently increase the demand for the lasers in the long-wavelength range (at 1.31 μm or 1.55 μm) exhibiting a superior cost performance. This is the reason VCSELs attract interests instead of the edge-emitting laser diodes, the yield of which is low.
In fact, VCSELs in the long-wavelength range have more problems than those in the short-wavelength range. Therefore, it is still impossible to replace the edge-emitting laser diodes.
There have been proposed structures of VCSEL in the long-wavelength range that exceeds 1 μm in the oscillation wavelength. One structure employs a GaInNAs-based material lattice matched to a GaAs substrate, and another structure is a hybrid structure that includes an InGaAsP-based material in lattice matched to an InP substrate and further includes either a semiconductor multi-film reflector or dielectric multi-film reflector of further another material.
When the VCSEL employs the GaInNAs-based material as disclosed in Japanese Patent Application Publication No. 10-303515, the GaInNAs-based material is used for a quantum well layer of active layer, whereas AlGaAs/GaAs-based material, which has proved the performance in the short-wavelength range, is used for the multi-film that forms a reflector, a spacer layer, a contact layer, or the like. Accordingly, VCSEL can be fabricated in a relatively small number of processes, that is, after the epitaxial growth is implemented on the GaAs substrate, the current funneling region and electrodes are formed thereon. In other words, it is convenient because the material used for the quantum well layer of the existent short-wavelength VCSEL is changed from GaAs to GaInNAs. Therefore, many studies and experiments have been made and this is the closest to practical use in the long-wavelength VCSELs.
On the other hand, when fabricating the long-wavelength laser diode in which the InGaAsP-based material is used for the quantum well layer, generally, the InP substrate is used to form the lattice matched to the afore-mentioned material. However, a multilayer has to be as thick as at least 50 periods in one of the reflectors to enhance the reflectance, when fabricating the VCSEL with the InGaAsP-based material. This is caused by the characteristics of the refractive index of the InGaAsP-based material, which does not largely change relative to the relative proportions, unlike the refractive index of the AlGaAs-based material. The film having a large number of periods increases the element resistance value and degrades the heat dissipation capacity. It is not desirable in view of the reliability. That is to say, it is considered difficult to fabricate the reflector almost completely reflective, namely, 99 percent at least, by epitaxially growing the InP substrate lattice matched, in the long-wavelength VCSEL having the InGaAsP-based material.
In order to address the afore-mentioned problem, in the hybrid structure with the InGaAsP-based material as disclosed in U.S. Pat. No. 5,835,521, the reflectors are separately fabricated from the neighboring region of the active region, and then are bonded together in a later process. This is called substrate fusion bonding. The substrate fusion bonding makes it possible to bond the semiconductor substrates that cannot form a lattice matched structure, enabling various applications.
In the hybrid structure, however, a discontinuous interface is created between the active region into which the current is injected and the reflectors. The discontinuous interface is low in distinctness as compared to the interface of crystal growth. The carriers that travel through the discontinuous interface are trapped to the level formed in the interface. In most cases, this results in the nonradiative recombination that the carriers are changed to thermal outputs. There are few cases where the reflectors are used for current injection.
Generally, another structure, known as intracavity type, is employed. This structure includes a current path that bypasses the reflectors. However, even in this case, the current funneling region is separately necessary. Accordingly, the techniques for the selective etch or tunnel junction are also utilized together.
Japanese Patent Application Publication No. 10-321952 and Japanese Patent Application Publication No. 2002-134835 disclose the VCSELs in which the tunnel junction region is arranged in series between top and bottom semiconductor Bragg reflectors. This eliminates the necessity of p-type semiconductor Bragg reflector having a high light absorption and high resistance. N-type semiconductor Bragg reflectors are used for top and bottom reflectors to reduce the threshold current or the like. U.S. Pat. No. 6,515,308B1 also discloses the structure in which the tunnel junction region is interposed between the top and bottom reflectors in the nitride-based VCSEL.
Additionally, M. H. M. Reddy et al., “Selectively Etched Tunnel Junction for Lateral Current and Optical Confinement in InP-Based Vertical Cavity Lasers,” Journal of Electronic Materials, Vol. 33, Pages 118-122, 2004 discloses the long-wavelength VCSEL in which the selective etch and the tunnel junction are combined. In the InP-based VCSEL, the currents in the lateral direction and lights are confined by using the selective etch. E. Hall et al., “Increased Lateral Oxidation Rates of AlInAs on InP Using Short-Period Super lattices,” Journal of Electronic Materials, Vol. 29, Pages 1100-1104, 2000 discloses the method for selectively oxidizing the super lattices and describes the oxidation rate in short-period super lattices of AlAs and InAs provided on InP.
It is to be noted that the structure having GaInNAs-based material, even if the material used for the quantum well active layer and the thickness thereof are controlled as much as possible, there is the problem in that it is difficult to increase the oscillation wavelength up to 1.31 μm, namely, zero dispersion of optical fiber, without sacrificing the reliability or electric or optical characteristics. Light absorption increases as the carrier concentration is increased in the p-type DBR layer, causing degradation in the luminous efficiency. There are drawbacks for practical use.
On the other hand, in the hybrid structure, the tunnel junction is electrically separated from the surrounding region to define the current injection region. On this account, the selective etch, selective oxidation, crystal regrowth, or the like are employed. Accordingly, the quality of this region decides the characteristics of the laser.
The selective etch, however, is not desirable in view of the reliability, with the exception of laboratory use, because the element partially has so-called voids. This is not desirable in view of reliability, except for experimental use. The selective oxidation has limitations in the material lattice matched to the InGaAsP-based material to be oxidized, namely, the material lattice matched to InP, For example, when InAlAs is used, thermal treatment is needed at high temperature for a long time and it is difficult for practical use. The buried structure with the crystal regrowth is most commonly used, yet in this technique, the element characteristics largely depend on the crystal performance of the uppermost surface, causing variations in the characteristics.
As described, there is no long-wavelength VCSEL that exhibits sufficient characteristics in the structure and fabrication method. Therefore, it is demanding the VCSEL that is excellent in mass production and easy in fabrication process.